1. Field of the Invention
The present invention relates to an integrated semiconductor laser element having a plurality of semiconductor lasers integrated therein, a semiconductor laser module using the same, and an optical transmission system.
2. Description of the Related Art
As a wavelength-tunable laser for DWDM (Dense Wavelength Division Multiplexing) optical communications, for example, an integrated semiconductor laser element has been disclosed (see, for example, Japanese Patent Application Laid-open 2003-258368). FIG. 15 is a schematic plan view of an integrated semiconductor laser element having a conventional structure. An integrated semiconductor laser element 200 includes a plurality of DFB (Distributed Feedback) lasers 71-1 to 71-n (n is an integer equal to or larger than 2) which have different oscillation wavelengths from one another, a plurality of optical waveguides 72-1 to 72-n, an optical coupler 73, and a semiconductor optical amplifier 74, where all the elements are monolithically integrated on a same substrate.
An operation of the integrated semiconductor laser element 200 is explained. First, a DFB laser selected from the DFB lasers 71-1 to 71-n is driven. An optical waveguide optically coupled to the driven DFB laser from the DFB lasers 71-1 to 71-n guides a laser light output from the driven DFB laser. The optical coupler 73 transmits the laser light guided by the optical waveguide, and outputs the guided laser light from an output port 73a. The semiconductor optical amplifier 74 amplifies the laser light output from the output port 73a, and outputs an amplified laser light from an output end 74a. 
The above-mentioned integrated semiconductor laser element is used as an optical transmitter in combination with an external modulator, for example, for a long-haul optical transmission in a DWDM optical communication network system.
In recent years, in order to broaden the transmission bandwidth, the symbol rate has been increased, for example, from 10 gigabits per second to 40 gigabits per second. Therefore, in order to prevent a decrease of the optical energy per symbol, there is a need for a higher-power integrated semiconductor laser element. Similarly, in order to broaden the transmission bandwidth, the modulation system has been multivalued from OOK, PSK, and the like to QPSK, 8PSK, and 16QAM. Due to such a multivalued system, a loss of the external modulator is increased, and therefore, in order to compensate for the increased loss, there is a need for a higher-power integrated semiconductor laser element.
Furthermore, in the DWDM optical communication network system, a large number of wavelength-tunable lasers are used, and therefore it is desirable that each of the wavelength-tunable lasers has low power consumption. Particularly, because a power used for cooling a device takes a large portion in the whole power consumption in the wavelength-tunable laser, in order to suppress the power consumption, it is desirable to use a wavelength-tunable laser that can operate in a high temperature condition.
However, if an even higher-power output is achieved in the conventional integrated semiconductor laser element with a simultaneously-layered structure on the same substrate, the wavelength dependency of the output intensity of the laser light changes, thereby increasing a deviation of the output intensity with respect to the wavelength.